The present invention relates to electrically operated friction devices and controls therefor, and more specifically to an electromagnetic self-adjusting clutch which incorporates a fail-safe feature whereby when the clutch is energized, no torque is transmitted and when the clutch becomes de-energized, maximum torque is transmitted from the input means to the output means.
Fail-safe electromagnetic clutch devices have found to be particularly desirable in applications where in case of an electrical power failure it is desirable to insure continuous running of the clutching device in order to complete a cycle or engage the clutch in order to bring a device to a complete standstill and maintain the standstill position. For example, in an electric lift-type truck it would be desirable in the case of an electrical system failure to stop the lift forks in the position that they attained prior to the electrical failure. This would insure that the lift with its heavy load would not drop to the floor in the case of an electrical power failure. Also, in an electric lift truck the fail-safe electromagnetic clutch can be used as a braking device when the truck stalls so that the truck would come to a complete standstill upon electrical power failure or if the engine should stall. Further usage of this type of clutch could be in an elevator where it would be necessary to stop the elevator in case of power failure. The clutch could also be used to control a furnace fan where in case of power failure, one would want the fan to engage and utilize maximum torque in case of an electrical power failure so that the furnace could be properly cooled after an electrical power failure occurs. A fail-safe type clutch is also highly desirable in some automotive applications where the clutch provides a cooling means to either an engine or a transmission. Here, in case of an electrical power failure, the engine or transmission would quickly overheat if there was no cooling available, therefore, the clutch attached with a cooling fan would provide cooling for the transmission even though the electrical system may be malfunctioning. In general, the fail-safe electromagnetic clutch could be used in any application where it would be desirable to temporarily continue an operation of a cycled device after the electrical system has malfunctioned. Likewise, the clutch would be applicable in situations where due to loss of electrical power a function would need to be preformed in order to insure or avoid a major breakdown or disaster.
Such application places particular requirements on the clutch device, in that, generally it must be simple and economically constructed and at the same time provide positive engagement and a high torque output in order to enable the system in which it is utilized to either complete its cycle safely or totally avoid continuation of a cycled event. The clutch in such a system may be either off or on for long periods of time or cycled on or off for short periods of time depending on the output power requirements. Therefore, the clutch must be sufficiently durable to withstand constant use with a minimum amount of wear. Although known electromagnetic clutches have proven to be successful in meeting some of the above requirements, the instant invention is concerned with the compact construction which enables operation of the clutch device to maintain maximum torque output without adverse were effects on the components and further, the invention provides an unlimited wear adjustment of the clutch surface without adversely affecting the force necessary to maintain the output torque.
Heretofore, there have existed at least two recognizable types of electromagnetic clutch construction, one herein called the "cone" variation and the other herein called the "disc" variation. In the cone type construction; inter-engaging friction surfaces between engageable rotatable clutch elements are conically shaped and by their inherent geometric configuration required a lower axial force to develop sufficient frictional locking of the working faces for the rotation of a driven member or for bringing a driven member to a complete standstill. In the disc type construction the inter-engaging friction surfaces are generally disposed normal to the axis of the engageable rotatable clutch elements and in instances where the flux path passes through the working faces, the armature pull is entirely axial. The disc construction is particularly advantageous due to the flexibility for providing large axially directed flux paths and thereby provide for a stronger clutch engaging force.
Other prior art designs are known which combine the desirable characteristics of the two known types of electromagnetic type construction mentioned above. One such design provides an armature ring element which has a generally "L" shaped radial cross section; the element has one annular pole piece with a frustro-conical face and another annular pole piece with a flat disc-like face disposed normal to the clutch axis. This design, however, has several disadvantages. For example, since the armature ring element was one piece, there was no means for compensating for wear of either pole piece. Further, with the above mentioned design, the conical friction surface must be made from a magnetic material. Yet another disadvantage results from this design in that the outer magnetic pole force is almost in the radial direction instead of the preferred axial direction.
Another design uses the same principle, that is, threading the conical pole piece to the disc-like pole pieces. This design adds yet one more disadvantage to those listed above in that by forcing the flux path to pass through the thread, there must necessarily be an even greater loss in the generated clutching force.
Fail-safe electromagnetic clutch devices heretofore have been generally of the disc type. Providing a self-adjusting cone type of fail-safe clutch presents the problem of maintaining the fail-safe feature independent of the wear on the frictional surfaces of the clutch. Generally, prior art designs provide conical frictional elements which move into engagement with the mutually engageable conical face on an output member of the clutch. The force required to engage the mutual frictional conical surfaces was a function of the wear exhibited at the mutually engageable surfaces. Therefore, as wear occurred at the frictional surfaces as well as wear occurred on the armature and pole faces, the fail-safe feature would be affected with respect to the amount of force required to pull the clutch into engagement. Generally, prior art designs require greater electromagnetic forces to engage the mutually engageable frictional torque transmitting surfaces as wear occurs at the surface. Further, adjustments for wear on such prior art devices caused adverse wear on the armature faces opposite the pole faces. This was a result of centrifugal force acting on the adjusting means. This centrifugal force caused the adjusting means to be somewhat delayed thereby causing the pole faces of the output member to come into contact with the armature while rotating for a sufficient duration of time to cause adverse wear on the armature face. The problems associated with the adjusting feature naturally affected the fail-safe feature and caused some concern over its effectiveness.